In veterinary parasitology the major ecological interest is in production ecology, which concentrates on those aspects of the interaction of host, parasite and environment which determine the size of the host's parasite population and thus the risk to the production process. Control aims at reducing this population by breaking the life cycle in two ways: 1. (a)at the host level — by anthelmintic treatment to eliminate the parasite and perhaps more importantly to remove the source of environmental contamination 2. (b) at the environmental level — by segregating susceptible hosts from the infective stage of the parasite. The application of control measures requires an understanding of the parasite population pattern in the host and in the environment, and of the factors influencing the population. If these factors can be separated out and quantified by the use of systems analysis then the host—parasite system can be analysed and modelled. Such a model could be used in simulation studies on control strategies, and to indicate productive lines of research. The adult parasite population in the host and egg production are largely controlled by immunological factors, in particular the peri-parturient rise in egg output, and these physiological factors are difficult to quantify. However, the development and transmission of the external larval population can be extensively subdivided and studied experimentally, and utilisable data is becoming increasingly available for modelling purposes. The key factors governing the infective population are temperature and moisture and these have been used to develop models of the population pattern in the external environment. More sophisticated models have been constructed to predict population size as well as pattern and this approach is likely to dominate future ecological studies.